1. Field of the Invention
The present invention relates to an improved extension ladder structure, and more particularly to an extension ladder apparatus, which is assembled and folded easily (i.e. telescoping) and securely, providing superior safety.
2. Description of the Prior Art
Accordingly, stepladders have become a requisite tool for casual use. For example, climbing tools such as a ladder or scaffolding enable people to readily process construction or obtain the objects at a high place.
A conventional stepladder 800, as shown in FIG. 1, is assembled by two side rails having a footpad respectively to prevent slippage, and is provided with parallel spaced step portions therein. The stepladder appears as an A-frame ladder during usage.
Stepladders are typically large and bulky; therefore, it is uneasy and inconvenient to carry them. Moreover, the storage of a folded stepladder may occupy large space, which makes the use of a stepladder inflexible.
Various extension ladder structures have been disclosed in U.S. Pat. Nos. 4,989,692, 5,492,430, 5,495,915, 2004/0195043A, 6,708,800, and 5,743,355 to improve the fixed stepladder as shown in FIG. 1A. However, the extension ladders in those applications still require respective control during folding and cannot achieve the automatic folding, which is not ideal for use.
To improve the shortcomings of existing extension ladders, the Chinese Patent No. 200620113407.6 named “Extension Ladder” has been issued on Apr. 29, 2006 and the Utility Model No. 899705 has been obtained.
Referring to FIG. 1B, which is the partial view of the extension stepladder in the Chinese patent No. 200620113407.6, after a locking mechanism within the first-length transverse step portion is pulled, with a tie rod plate 801 that can be held up and down, one end of the tie rod plate 801 extends to the upper end of the cross hole, the out-protruding part is inserted into the lower end cross hole 802 of the upper transverse step portion, and then the turning rod 803 disposed within the upper transverse step portion is pushed so that the locking bar withdraws from the upper transverse step portion to unlock. Accordingly, the locking mechanism above the second length can be unlocked in order and each length of the side rail sections will go down and be folded.
However, after the Chinese Patent No. 200620113407.6 described above has been completed, through constantly tests, it is found that when an extension ladder is used in a dirty environment, the holes above or under the step portions may be filled with dust and other contaminants so that the tie rod plate cannot be moved smoothly, which will cause unlock failure or other problems.
Also, unlocking requires the tie rod plate 801 to push the turning rod 803. If a tie rod plate 801 gets stuck or cannot pass through the cross hole, the entire stepladder cannot be folded smoothly which causes inconvenience while using it.
As to the side rail sections that can be fit in with each other, since there is no effective locking mechanism, the upper side rail section may depart from the lower side rail section when they are used erroneously, which causes unsafety.
Accordingly, the present invention has been invented to solve the above-mentioned problems occurred in the prior art.